Mutations in mitochondrial DNA lead to a spectrum of neurodegenerative diseases for which no effective treatment exists.This is a group of untreatable disorders affecting the eye, nervous system and heart, some clinically characterized over a century ago, but they are now known to be a spectrum of molecularly defined diseases. We had chosen to start with one of the most severe, the ATP6 mutation (T8993G) in mitochondrial (mt) DNA responsible for Maternally Inherited Leigh Syndrome (MILS), a neurologic disease renowned for causing blindness and rapidly fatal encephalomyelopathy in early childhood or Neurogenic Ataxia and Retinitis Pigmentosa in early adulthood. We have made major strides towards determining the pathogenesis and testing treatments for another common mitochondrial disorder Leber hereditary optic neuropathy (LHON). In this application, we propose to design, modify and test the efficacy and safety of a clinically relevant vector for the treatment of MILS and NARP caused by mutated ATP6. Our Aims are: (1) To facilitate translational studies for MILS and NARP by developing an MTS AAV serotype 9 vector for intravenous use to deliver the normal ATP6 gene directly to the mitochondria and test expression to rescue respiration in cybrid cells with 100% mutated T8993G mitochondrial DNA. (2) To evaluate biological effects of intravenous delivery of MTS AAV9 vectors in normal mice that result in mitochondrial gene transfer without adverse effects.(3) To rescue visual loss, encephalomyelopathy and death in transgenic ATP6 mice. We hope to identify the conditions for long-term rescue of visual loss and encephalomyelopathy in mice, so that this approach can be tested in a phase I/II clinical designed to restore visual and neurologic function in MILS and NARP patients.